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Abstract:

A measuring method of chromium-free coating film thickness on surface of
strip steel comprising selecting two water-soluble chemical substances
containing elements P, Ca, Ti, Ba or Sr and not reacting with a
chromium-free coating liquid; adding the chemical substances into the
chromium-free coating liquid and agitating them to be homogeneous,
thereafter, fabricating a reference sample of coating film; using a ray
emitted by an off-line film thickness instrument to excite the two
water-soluble chemical substances so as to obtain characteristic
spectrums to obtain a correction function expression between the measured
film thickness and the thickness correction value by fitting; adding the
water-soluble chemical substance which has a weak characteristic spectrum
into a chromium-free coating liquid, and using the expression to obtain
the actual coating film thickness. The method is capable to monitor film
thickness with no adverse effect on adhesiveness and corrosion-proof of
the coating film.

Claims:

1. An on-line detection method of chromium-free coating film thickness on
surface of strip steel, characterized in that the method comprises the
following steps: Step 1: selecting two water-soluble chemical substances
which contain elements P, Ca, Ti, Ba or Sr and do not react with a
chromium-free coating liquid; Step 2: adding the two water-soluble
chemical substances selected in step 1 into the chromium-free coating
liquid and agitating them to be homogeneous, thereafter, fabricating a
reference sample of the coating film; Step 3: using a ray emitted by an
off-line film thickness detection instrument to excite the two
water-soluble chemical substances so as to obtain characteristic spectrum
of the two water-soluble chemical substances, thereby to have the coating
film thickness of the reference sample; the coating film thickness
determined by means of the water-soluble chemical substance which has an
intensive characteristic spectrum is taken as actual film thickness,
whereas the coating film thickness determined by means of the
water-soluble chemical substance which has a weak characteristic spectrum
is taken as measured film thickness, the difference between the actual
film thickness and the measured film thickness is taken as a thickness
correction value; by many times of such operation, obtaining many
thickness correction values corresponding to the many measured film
thickness, a correction function expression between the measured film
thickness and the thickness correction value is obtained by fitting
thickness correction values and measured film thickness; Step 4: adding
the water-soluble chemical substance which has a weak characteristic
spectrum into the chromium-free coating liquid, and using a ray emitted
by an on-line coating film thickness instrument to excite the substance
and to obtain the measured film thickness, and then using the correction
function expression to obtain a thickness correction value, finally,
actual coating film thickness is obtained from the measured film
thickness and thickness correction value.

2. The on-line detection method of chromium-free coating film thickness
on surface of strip steel in claim 1, characterized in that the
water-soluble chemical substances which will not react with the
chromium-free coating liquid are β-sodium glycerol-phosphate,
calcium acetate, titaniumocene complexes, barium chloride or strontium
acetate.

3. The on-line detection method of chromium-free coating film thickness
on surface of strip steel in claim 1, characterized in that the ray is
χ ray or γ.

Description:

FIELD OF THE INVENTION

[0001] This invention relates generally to measurement of coating film
thickness on surface of strip steel, and particularly, to a measuring
method of chromium-free coating film thickness on surface of strip steel.

BACKGROUND OF THE INVENTION

[0002] In the situation that environment protection is increasingly
concerned, and the voice for the chromium-free green product is stronger
and stronger. The major steelworks in the world are all active to develop
chromium-free coatings to substitute chromate coatings for the film of
their products.

[0003] To apply chromium-free coating film onto the surface of strip steel
is rather difficult because there are not good methods and standards for
both preparation of chromium-free coating liquid and detection of
chromium-free coating film thickness. Especially for large-sized electric
machines, after a number of laminates of strip steel sheet is stacked
together, uneven thickness of coating film of a laminate or laminates
will result in an uneven thickness of the whole stack of laminates.
Therefore, how to accurately detect coating film thickness and how to
optimize application process of coating film are both stiff challenges in
production of strip steels with chromium-free coating film.

[0004] In the case of on-line detection of coating film thickness, the
prior arts often use ray inspection methods which inspect characteristic
elements. Or the prior arts use X ray fluorescence remitter to perform
the on-line or off-line detection. In these detection methods, however,
it is necessary for a coating liquid to contain some characteristic
element, such as chromium, which is easy to emit X-ray fluorescence when
it is excited, and the content of the element must be changeless.
Therefore, there are only very few coating films whose thickness can be
detected by use of these methods, and each set of detection instrument
can detect only one characteristic element and so the thickness of only
one type of coating film. For producing various types of different-type
coating films, it is necessary to equip with multiple sets of thickness
detection instrument, which is inconvenient to use in mass production.
Even if a type of coating film is kept unchanged, the relative difference
of compositions of different batches of coating films still has a
non-negligible effect on detection result. Chromium-free coating film
does not contain characteristic element Cr, so those methods are helpless
for on-line detecting.

[0005] Nowadays, thickness of chromium-free coating film is often detected
by means of off-line film thickness detection instruments. The detection
instruments largely apply the eddy current principle to carry on
detection. It takes specimens from the head end and the tail end of a
steel strip and measures thickness at multiple points on the work side,
middle side, and the driven side, and then takes an average of all the
measurements as the thickness of the whole coating film on the steel
strip. The advantages of those methods lie in that it is convenient to
carry out measuring at many points; however, in those methods, it is
necessary to have a typical sample. In addition, because the off-line
detection systems themselves and the film thickness reference card both
have rather big errors and it is impossible to implement on-line monitor,
those methods are unsuitable to manufacture steel strips through a
continuous annealing furnace and are prone to bring about an uneven film.

SUMMARY OF THE INVENTION

[0006] The objective of the invention is to provide an on-line detection
method of chromium-free coating film thickness on surface of strip steel.
The method is to add weak characteristic element into chromium-free
coating liquid to obtain a correction function and further to obtain the
thickness of the coating film, which does not has a correct measured
result, but only has no effect on adhesiveness, corrosion-proof and
environmental performance of the coating film.

[0007] The objective is achieved by a on-line detection method of
chromium-free coating film thickness on surface of strip steel comprises
the follows:

[0009] Step 2, Adding the two water-soluble chemical substances selected
in Step 1 into the chromium-free coating liquid and agitating them to be
homogeneous, thereafter, fabricating a reference sample of the coating
film;

[0010] Step 3, Using a ray emitted by an off-line film thickness detection
instrument to respectively excite the two water-soluble chemical
substances so as to obtain characteristic spectrums and to have the
thickness of the reference film sample; the coating film thickness
determined by means of the water-soluble chemical substance which has an
intensive characteristic spectrum is taken as actual film thickness,
whereas the coating film thickness determined by means of the
water-soluble chemical substance which has a weak characteristic spectrum
is taken as measured film thickness, the difference between the actual
film thickness and the measured film thickness is taken as a thickness
correction value; via many times of such operation, it is to obtain many
thickness correction values corresponding to the many measured film
thickness, and a correction function expression between the measured film
thickness and the thickness correction value can be obtained by fitting
those discrete data;

[0011] Step 4, Adding the water-soluble chemical substance which has a
weak characteristic spectrum into the chromium-free coating liquid, and
using a ray emitted by an on-line coating film thickness detection
instrument to excite the water-soluble chemical substance and so to
obtain the measured film thickness, and then using the correction
function expression to obtain a thickness correction value, finally,
actual coating film thickness can be obtained from the measured film
thickness and thickness correction value.

[0012] The water-soluble chemical substances which will not react with the
chromium-free coating liquid are β-sodium glycerol-phosphate,
calcium acetate, titaniumocene complexes, barium chloride or strontium
acetate;

[0013] The ray is χ ray or γ ray. The on-line detection method
of chromium-free coating film thickness on surface of strip steel, by
means of adding weak characteristic element into chromium-free coating
liquid to obtain a correction function is capable to detect thickness of
the coating film in time, and to by means of on-line detection, is
capable to monitor film thickness effectively and optimize the coating
process continuously; and in the meantime, the method is applicable to
different-type chromium-free coating liquids with high precision, and has
no adverse effect on adhesiveness, corrosion-proof and environmental
performance of the coating film

BRIEF DESCRIPTION OF DRAWINGS

[0014] FIG. 1 is a curve graph of a function of the measured thickness and
the correcting thickness in embodiment 1 of the detection method of
chromium-free coating film thickness on surface of strip steel of the
invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The invention is now described in detail with a specific
embodiment. It is understood that the embodiment is used only for
explaining the invention but not for limiting the scope of the invention.
It is also understood that person skilled in the art can make various
changes or modifications without departing from the spirit and scope of
the invention after having read the description herein, therefore any
versions identical or equivalent to the solution herein all fall in the
scope defined in the claims of the invention.

Embodiment 1

[0016] a detection method of chromium-free coating film thickness on
surface of strip steel comprises the following steps:

[0018] Step 2: adding the two chemical substances selected in step 1 into
the chromium-free coating liquid and agitating them to be homogeneous,
thereafter, fabricating a reference sample of the coating film;

[0019] Step 3: using χ ray or γ ray emitted by an off-line film
thickness instrument to excite the two chemical substance so as to obtain
characteristic spectrums, thereby to have the coating film thickness of
the reference film sample; the coating film thickness determined by means
of the additive which has an intensive characteristic spectrum can be
taken as actual film thickness H, whereas the coating film thickness
determined by means of the additive which has a weak characteristic
spectrum is taken as measured film thickness h, the difference between
the actual film thickness H and the measured film thickness h is taken as
a thickness correction value ΔA; through many times of such
operation, it is possible to obtain many thickness correction values
corresponding to the many measured film thickness; a correction function
expression between the measured film thickness h and the thickness
correction value ΔA can be obtained by fitting those discrete data.

[0020] Step 4: adding the additive which has a weak characteristic
spectrum into the chromium-free coating liquid, and using a ray emitted
by an on-line coating film thickness instrument to excite the additive
and to obtain film thickness, and then using the film correction function
expression to obtain a thickness correction value, finally, actual
coating film thickness can be obtained from the measured film thickness
and thickness correction value.

[0021] In the embodiment, for fabricating the reference film sample, the
chemical substances, i.e. calcium acetate ((CH3COO)2Ca.H2O) and
β-sodium glycerol-phosphate (C3H7Na2O6P.5H2O), which contain Ca, P
element, are added by 0.1˜1%, by weight percent, into the
chromium-free coating liquid; the characteristic spectrums of the two
elements are detected by use of a off-line film thickness instrument,
thereby the measured film thickness and thickness correction value are
obtained which are shown in Table 1.

[0022] The curve shown in FIG. 1 is plotted based on the data in Table 1,
thereafter, a function expression is obtained by fitting the curve:
thickness correction value

ΔA=0.0600+0.0325h (1)

[0023] In mass production of the chromium-free coating film, coating film
thickness is detected by using a ray emitted by a on-line film thickness
instrument to excite element P, wherein the measured film thickness on
the upper surface is 3.16 g/m2, and the measured film thickness on
lower surface is 3.1 g/m2.

[0024] Substituting h in expression (1) with the measured film thickness
on the upper surface and the lower surface, respectively, the actual film
thickness on the upper surface and the lower surface is obtained
respectively: 3.32 g/m2 and 3.26 g/m2.

[0025] By comparing between the properties of the coating films of the
steel sheets with and not with β-sodium glycerol-phosphate
(C3H7Na2O6P.5H2O), it is concluded that to add β-sodium
glycerol-phosphate (C3H7Na2O6P.5H2O) has not adverse effect on film
properties such as adhesiveness, corrosion resistivity, and so on, as
shown in Table 2.